NASA Technical Reports Server (NTRS) 19700000250: Mechanism and kinetics of aging in Inconel 718

June 1970

Brief 70-10261

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Mechanism and Kinetics of Aging in Inconel 718

Inconel 718 is a nickel-chromium-iron-molybdenum
alloy, hardened by the addition of columbium, tan-
talum, aluminum, and titanium. It is an age-harden-
able superalloy with good high-temperature strength
and a rather sluggish aging response. The low rate
of precipitation hardening exhibited during aging is a
major factor in giving this alloy good weldability.
Its property of slow aging facilitates studies concerned
with the precipitation process.

Age hardening in Inconel 718 was investigated
using Brinell hardness measurements. Aging isotherms
were determined at temperatures of 1141°, 1252°,
1314°, 1390°, and 1492°F. On aging at the three low-
est temperatures, the hardness increased to a maxi-
mum and then remained constant. The aging time to
reach maximum hardness was over 1000 hr at 1141°F,
about 200 hr at 1252°F, and only 35 hr at 1314°F. At
1390° and 1492° F the hardness increased to a maxi-
mum within a few hours, then decreased. Maximum
hardness was lower and aging time to reach maximum
hardness was longer at 1492°F than at 1390°F.

The formation of a precipitate, identified as colum-
bium-rich y Ni .3 (Cb, Al, Ti), causes an increase in
hardness. The precipitate has a face-centered cubic
structure, and forms coherently with the matrix during
aging.

An aging mechanism was proposed whereby the
observed changes in hardness below about 1340°F
were associated with the formation and loss (by over-
aging) of the 7 ' phase. An activation energy of 76,500
calories per mole was determined for the growth of 7 '.

There is an incubation period before Inconel 718
increases in hardness. The shortest incubation period
corresponds to the fastest nucleation rate of the co-
herent 7 ' phase and occurs at approximately 1340°F.
Notes:

1. This information maybe of value in the formulation
of new alloys using aging mechanisms.

- 2. Requests for further information may be directed
to:

Technology Utilization Officer
Marshall Space Flight Center
Huntsville, Alabama 35812
Reference: TSP70- 10261

Patent status:

No patent action is contemplated by NASA.

Source: B. G. Koepke, R. J. Zeto,
and W. E. Hensley of
North American Rockwell Corporation
under contract to
Marshall Space Flight Center
(MFS-18775)

Category 04

This document was prepared under the sponsorship of the National
Aeronautics and Space Administration. Neither the United States
Government nor any person acting on behalf of the United States

Government assumes any liability resulting from the use of the
information contained in this document, or warrants that such use
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